⁶US environmental agencies today typically use ppb (parts per billion), while international and European agencies typically use μg∕m³, with 40μg∕m³ the current WHO annual average recommended threshold (about 19.5 ppb under given assumptions about air pressure and temperature). Air pollution in Boston has declined by over half since the 1970’s, but in our data set, all the TASSIM zones had values between two and four times today’s international threshold for average annual NOX exposure.